The present invention relates to a gas turbine including a ceramic turbine and a metallic impeller, and more particularly to a joint structure of a ceramic turbine to a metallic impeller.
A joint structure as shown in FIG. 7 is known from Laying-open Japanese Patent Application No. 54-42520. FIG. 7 shows a ceramic turbine 101 with an integral shaft 102 received in a cylindrical bore of an enlarged diameter portion 106 of a metallic impeller shaft 103. Ring seal groove 104 and oil groove 105 are formed on the outer surface of the enlarged diameter portion 106. Brazing filler metal 107 is distributed between the closely fitted surfaces of the ceramic shaft 102 and the enlarged diameter portion 106. FIG. 7 also shows bearings 108 and 109 and a threaded portion 103a with which an impeller is fixedly carried by the impeller shaft 103.
A joint structure shown in FIG. 8 is known. FIG. 8 shows a ceramic turbine 111 including an integral ceramic shaft 112 formed with a ring seal groove 114 and an oil groove 115, and a metallic impeller shaft 113 having a flanged end 113a. A brazing filler metal 117 is disposed between the closely fitted surfaces of an axial end 112a of the ceramic shaft 112 and flanged end 113a. FIG. 8 also shows bearings 118 and 119, and a threaded portion 115a formed on the metallic impeller shaft 113.
According to the known joint structure shown in FIG. 7, the brazing filler metal 107 distributed between the closely fitted surfaces of the metal and ceramic extends axially toward the open end of the cylindrical bore beyond a predetermined plane where the ring seal groove 104 lies as shown in FIG. 9. Thus, the brazing filler metal 107 has a portion 107a which is always exposed to high temperature exhaust gas so that it is softened and/or melted, and oxidized, resulting in considerable drop in joint between the closey fitted surfaces of the ceramic shaft 102 and enlarged diameter portion 106.
According to the other known joint structure shown in FIG. 8, since the ceramic turbine 111 and the metallic impeller shaft 113 are joined by welding between the axial end 112a of the ceramic shaft 112 and the flanged axial end 113a, there is the limit to reliability in strength of the joint. Besides, since the ceramic shaft 112 is formed with the ring seal groove 114 and oil groove 115, there would occur leakage of oil if the ceramic shaft portion 112 should damage.
Accordingly, an object of the present invention is to provide a joint structure wherein the brazing filler metal distributed between the closely fitted surfaces of the ceramic shaft and the metallic shaft is not exposed to high temperature exhaust gas and the leakage of oil would not occur even if the ceramic shaft portion should be damaged.